THE SEA ANEMONES AND CORALS 



3473 



which secrete coral are generally simpler, and smaller, and grow in vast colonies. It 

 is the accumulation of all their little contributions of coral which, in the process of 

 time, build up islands or even continents. In regarding coral animals as reef builders, 

 we may leave out of account, as unimportant, those which form hard spicules within 

 their bodies, and consider only those which perform most of the work. Imagine a 

 crowd of small animals like sea anemones fixed to a rock, each one secreting a layer 

 of carbonate of lime between itself and the rock, and this layer becoming thicker 

 and thicker till each polyp rises on a little pedestal. There is probably a race 

 between them, as there is among trees in a forest which shall reach the highest 

 to get most of the food as it passes by on the currents in the water. Now, it is 

 obvious that a crowded colony of polyps like this would in a short time add a 

 thick layer of solid carbonate of lime to the rock on which they first settled, and 

 this is, in brief, the principle of reef building. As a matter of fact, however, it is 

 not quite so simple. The layer each polyp secretes is not a smooth flat disc, evenly 

 secreted by the whole surface of its foot. Some parts of the foot secrete more than 

 others, hence those parts rise up as spines, walls, and rings, which protrude into the 

 body of the polyp without, however, breaking through the skin. These probably 

 help to fasten the polyp to its pedestal, and prevent it from being swept off by strong 

 currents. The figure on p. 3479 is a good illustration of one of these plates. Each 

 genus of corals has a pattern of its own, each one perfect and beautiful in its way, 

 and it is frequently a puzzle to discover how it is made. When a crowd of polyps 

 grow in contact, their pedestals will also grow in contact and form continuous 

 masses; this growing in contact being insured by their ordinary method of multipli- 

 cation. For a coral polyp does not have to wait until another takes up a position 

 beside it, but as soon as it can feed freely, it begins to bud or divide, producing a 

 number of young polyps close around it. These also bud 

 in their turn and are soon surrounded by young polyps, 

 and in this way such compact colonies are formed that it is 

 a struggle among the inner ones to avoid being suffocated. 

 We thus have densely crowded colonies of polyps strug- 

 gling upward, each individual secreting a more or less 

 beautiful and complicated pedestal. The pedestals are 

 fused together in a hundred different ways, and from these 

 different patterned pedestals, with their various ways of 

 fusing together, are produced the almost countless different 

 kinds of coral which together build up coral reefs. 



In a growing polyp stock the individuals usually re- 

 main in organic connection, that is to say, each first pro- 

 vides for itself, and then shares its superfluity with the 

 others, sometimes by means of a continuous reticulated 

 system of canals running from polyp to polyp, perforating 

 the stony substance which often separates the members of 



the one stock from another. The whole stock may thus be physiologically one 

 creature with many mouths. Where, however, the secretion of the pedestal is very 

 rapid and the budding very slow the polyps may separate, each standing at the end 

 218 



OUTLINE OF Caulastrcza. 

 (Natural size.) 



